This invention relates to a membrane keyboard assembly for use in a membrane keyboard switch device.
There have been proposed and practically employed a variety of membrane keyboard assemblies for use in a membrane keyboard switch device and in most of the conventional membrane keyboard assemblies, the flexible base has connection lines and contact areas formed thereon by applying electrically conductive ink to the base by the screen printing. As the electrically conductive ink to be used for the purpose, silver paint has been generally employed. However, the silver paint oxidizes easily and also presents a serious migration problem and thus it has been generally followed that resistive layers are applied to the connection lines and contact areas on the base by the screen printing of resistive paint. As the resistive paint, carbon paint has been generally employed. Although the carbon paint has a relatively low electric conductivity and a relatively high inherent resistance because of its characteristic properties, the carbon paint has an excellent stability in its resistance value after the resistance value has been initially set.
The membrane keyboard assembly functions to open and close the switch in which the membrane keyboard assembly is incorporated as its component when the membrane keyboard assembly is depressed down by a human finger. However, at the time of depressing down, the membrane keyboard assembly has a high potential possibility to receive static electricity from a human body or other objects. Therefore, the membrane keyboard switch device employing the membrane keyboard assembly as its component is provided with an external grounding circuit such as a resistance or capacitor as a discharge means when the membrane keyboard assembly receives such static electricity.
As shown in FIG. 13 of U.S. Pat. No. 4,145,584, for example, the discharge resistance is electrically connected to a line outside of the membrane keyboard assembly. However, in such an arrangement, there is the disadvantage that the cost for purchase of the resistance itself and that for connecting the resistance to the line are added to the cost of the membrane keyboard switch device employing the membrane keyboard assembly resulting in an expensive membrane keyboard switch device. In addition, such an arrangement has the disadvantage that the switch device inevitably becomes a large size device.